"Draw a picture to show the problem then write a division sentence. Bobby has 12 model cars. He puts them in cases. Each case holds 4 cars. How many cases does he fill?" My son drew 3 boxes with four dotsin each and wrote 12/3 equals 4 and answered that bobby filled 3 cases. Shouldn't it be the other way around?

My students think PEMDAS means that all addition should be done before
any subtraction is done. How can I show and convince them that they
need to do both operations at the same time working from left to right?

To get the difference by the missing addend method we are not supposed
to change the given problem into subtraction of one subtrahend, but to
work out the problem as it is. We are not even to use regrouping.